In a high intensity discharge (HID) lamp, a medium to high pressure ionizable gas, such as mercury or sodium vapor, emits visible radiation upon excitation caused by passage of current through the gas. One class of HID lamps comprises inductively-coupled electrodeless lamps which generate an arc discharge by generating a solenoidal electric field in a high-pressure gaseous lamp fill. In particular, the lamp fill, or discharge plasma, is excited by radio frequency (RF) current in an excitation coil surrounding an arc tube. The arc tube and excitation coil assembly acts essentially as a transformer which couples RF energy to the plasma. That is, the excitation coil acts as a primary coil, and the plasma functions as a single-turn secondary. RF current in the excitation coil produces a time-varying magnetic field, in turn creating an electric field in the plasma which closes completely upon itself, i.e., a solenoidal electric field. Current flows as a result of this electric field, resulting in a toroidal arc discharge in the arc tube.
A practical excitation coil configuration avoids as much light blockage by the coil as possible in order to maximize light output. An exemplary coil configuration is described in commonly assigned U.S. Pat. No. 5,039,903 of G. A. Farrall, issued Aug. 13, 1991 and incorporated by reference herein. However, even though the excitation coil configuration of the Farrall patent is effective in avoiding much light blockage from the lamp, it is desirable to provide a new lamp configuration which increases light output even further.